German Publication DE 38 34 555 C1 discloses a fluid friction coupling for optionally connecting or blocking the differential effect of the drive of the driving wheels of a motor vehicle. This fluid friction coupling comprises a first coupling part provided in the form of a hub and a second coupling part which is rotatable relative to said first coupling part and which is provided in the form of a housing. The hub and housing, jointly, form a sealed annular chamber in which inner plates and outer plates are alternately received in the longitudinal direction. The inner plates are connected to the hub in a rotationally fast and axially displaceable way. The outer plates are loosely arranged between supporting rings received in the housing in a rotationally fast and axially displaceable way. By activating an operating device, there is generated an axial force so that the supporting rings are axially pressure-loaded, thus achieving a friction-locking effect between the supporting rings and the plates received therebetween. In this way, the torque transmitting capacity can be controlled and adapted to the respective desired operating conditions. A disadvantage of this fluid friction coupling for controlling the torque transmitting capacity is that as a result of the large number of components, its assembly is relatively complex.
German Publication DE 199 18 411 A1, discloses a controllable viscous coupling. In contrast to the above coupling, the annular chamber contains three functionally different sets of coupling plates of which each inner set comprises first plates and each outer set comprises second plates. The plates of the first set are connected in a rotationally fast way to the respective coupling part. To adapt the viscous coupling to different driving conditions of the vehicle, the two remaining sets comprising different diameters are provided with coupling discs for connecting or disconnecting the outer second plates to or from the housing. In this way, the torque transmitting capacity can be adapted more accurately to the existing operating conditions.